1. Field of the Invention
The invention relates to a nonvolatile semiconductor memory device, and to a method for controlling a threshold voltage thereof to read and/or write data therefrom.
2. Description of the Related Art
Generally, an EEPROM having a floating gate is well known as the nonvolatile memory from and to which data can be electrically read and written. The EEPROM includes a silicon substrate, a source and a drain in which impurities are diffused, a channel area, a gate oxide layer formed on the channel area, and a control gate. The floating gate is formed on the gate oxide layer, and the control gate is formed on the floating gate.
In the EEPROM, information is stored by injecting and storing electrons at the floating gate. A uniform condition, such as a writing voltage or a writing time, is applied to all memory cells to where date is written to memory cells in the EEPROM. Therefore, if every memory cell does not have the same characteristics, the memory cells do not have a uniform a threshold voltage. Here, the voltage of the control gate at the time that the channel between the source and the drain is formed is defined as the threshold voltage.